1. Field of the Invention
The present invention relates to apparatus for purifying exhaust gas such that particulate matter material such as soot in the exhaust gas of an internal combustion engine such as a diesel engine is trapped by a filter.
2. Description of the Related Art
Recent development of transportation means is remarkable, and travel over a long distance has become part of our everyday life. On the other hand the development has had unfavorable effects on the global environment. In particular, air pollution caused by the exhaust gas of internal combustion engines is presently one of the greatest problems. In Japan, the air of large cities such as Tokyo, Nagoya, and Osaka is so polluted that people hesitate to open the windows of their driving cars. Air pollution is said to be a cause of lung cancer. It is a problem human beings must solve.
In order to keep the air pollution at a minimum level, cleaning of the exhaust gas of internal combustion engines has been developed.
As a prior apparatus for cleaning exhaust gas, there is, for example, one described in SAE (Society of Automotive Engineers: The Engineering Society For Advancing Mobility Land Sea Air and Space; registered trade mark) P-240, Jan. 25, 1991, pp 83-92. FIG. 12 shows the prior apparatus for purifying exhaust gas. As shown in FIG. 12, in the prior apparatus, the exhaust gas from a diesel engine 81 is first muffled by a muffler 82 and then purified by a filter 83. The filtering function of filter 83 is reduced after its use for a certain time period, during which carbon or soot components adhere to the filter. Therefore, in the prior apparatus, an exhaust pressure sensor 84 detects the malfunction of a filter. Then a controller ECU 89 closes a valve 85 and opens a valve 86 to discharge the exhaust gas through a by-pass pipe 87, while heating up filter 83 by an electric heater 88 installed in the filter to burn up carbon components trapped in filter 83 and to reactivate the filter.
In this type of exhaust gas purifying apparatus, the filter is made of ceramic material. Table 1 shows characteristics of main ceramic materials used for exhaust gas filters, and Table 2 shows characteristics of typical particulate filters.
TABLE 1 ______________________________________ Maximum Thermal Melting working expansion Ceramic point temperature coefficient Materials (.degree.C.) (.degree.C.) (.times.10.sup.-6 /.degree.C.) ______________________________________ Silica 1730 900 0.5 (SiO.sub.2) Cordierite 1465 1000 1.5 (2MgO .multidot. 2Al.sub.2 O.sub.3 .multidot. 6SiO.sub.2) Mullite 1840 1200 5.0 (3Al.sub.2 O.sub.3 .multidot. 2SiO.sub.2 Alumina 2045 1500 8.8 (Al.sub.2 O.sub.3) Zirconia 2580 1600 10.0 (ZrO.sub.2) ______________________________________
TABLE 2 ______________________________________ Fiber Ceramic Ceramic ceramic honeycomb foam Wire mesh ______________________________________ Composi- Mullite Cordierite Cordierite Stainless tion Steel Volume 0.3 .about. 0.4 1.6 .about. 1.8 0.3 .about. 0.4 -- density (g/c ml) Vacancy 85 .about. 92 35 .about. 50 84 .about. 88 90 .about. 98 rate (%) Maximum 1200 1000 1000 700 working tempera- ture (.degree.C.) Thermal 5 .times. 10.sup.-6 1 .times. 10.sup.-6 1 .times. 10.sup.-6 -- expansion coeffi- cient (/.degree.C.) Thermal 0.05 0.70 0.70 -- conduc- tivity (Kcal /mh .degree.C.) Thermal 1000 600 800 -- shock resist- ance (.degree.C.) Against Stable Stable Stable -- chemicals ______________________________________
As seen from these tables, Ceramics are very weak against a rapid change of temperature. Therefore, when the engine is working at a high load while after by-passing the exhaust gas, the controller switches the valves to force the exhaust gas to flow into the filter, then exhaust gas at over 500.degree. C. flows into the filter, and the filter is heated rapidly and cracked by the rapid change of temperature, so that the filter will not properly function.
FIG. 13 shows time/temperature characteristics inside a filter when the prior engine switches the valves during its work at a high load to force the exhaust gas to flow into the filter. In FIG. 13, each line shows each of outputs of plural temperature sensors arranged at various positions in the filter. As is clearly shown in FIG. 13, the internal temperature of the filter rises upto 500.degree. C. or more within 30 seconds or so and this rapid temperature rise causes cracks in the ceramic material used for the filter. In fact, many cracks were observed in the filter when inspected after the experiment.